A computer algorithm is being developed to count the number of alleles that siblings and half siblings have in common, after the mother's alleles are subtracted from each child. These allele counts are averaged for each pair of people in question. Deviations from expected average values give an indication of problems in the pedigree structure (e.g., non-paternity, mislabeled DNA samples) when a large number of genetic markers are tested. The method has been used to examine 60 JP pedigrees with favorable preliminary results. When conducting linkage analysis studies using family pedigrees, it is important that the family structure is accurately reported, in order to detect linkage between an inherited disease and genetic marker. To improve our ability to verify family structures, we have developed a new strategy to evaluate allele sharing among family members.